Air-stable surface-passivated perovskite quantum dots (QDS), methods of making these QDS, and methods of using these QDS

ABSTRACT

Embodiments of the present disclosure provide for passivated quantum dots, methods of making passivated quantum dots, methods of using passivated quantum dots, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/773,680, which was filed on May 4, 2018, and issued as U.S. Pat. No.10,927,295, which claims the priority to International PCT ApplicationNo. PCT/IB2016/056715, filed on Nov. 8, 2016, which claims the benefitof and priority to U.S. Provisional Application Ser. No. 62/252,525,having the title “AIR-STABLE SURFACE PASSIVATED PEROVSKITE QUANTUM DOTSFOR ULTRA-ROBUST, SINGLE-AND TWO-PHOTON-INDUCED AMPLIFIED SPONTANEOUSEMISSION,” filed on Nov. 8, 2015 and U.S. Provisional Application Ser.No. 62/335,727, having the title “AIR-STABLE SURFACE-PASSIVATEDPEROVSKITE QUANTUM DOTS (QDS), METHODS OF MAKING THESE QDS, AND METHODSOF USING THESE QDS,” filed on May 15, 2016, each of the disclosures ofwhich are incorporated herein in by reference in their entirety.

BACKGROUND

Lead halide perovskites have recently emerged as promising candidatematerials for optoelectronic applications due to their size-tunableoptical bandgaps, attractive absorption, narrow emission, andextraordinary charge transport properties. These impressivecharacteristics have also triggered intense interest in applyingperovskites to the field of light-emitting diodes (LEDs). However,perovskite LEDs (PeLEDs) still exhibit overall low performance limitedstability.

SUMMARY

Embodiments of the present disclosure provide for compositions ofpassivated perovoskite quantum dots, methods of making passivatedperovoskite quantum dots, and the like.

An embodiment of the present disclosure includes, among others, acomposition comprising a passivated perovskite quantum dot, wherein thepassivated perovskite quantum dot is of the form APbX₃, where A is Cs⁺,Rb⁺, CH₃NH₃ ⁺, or HC(NH₂)₂ ⁺, and X is a halogen, wherein the passivatedperovskite quantum dot includes a capping ligand comprised of aninorganic-organic hybrid ion pair. In an embodiment, theinorganic-organic hybrid ion pair is a sulfur based inorganic-organichybrid ion pair or a halide based inorganic-organic hybrid ion pair. Inan embodiment, the passivated perovskite quantum dot is selected fromthe group consisting of: CsPbCl₃, CsPbCl_(3-x)Br_(x) (x is 0 to 3), andCsPbBr₃. In an embodiment, the passivated perovskite quantum dot has thecharacteristic of being able to have an amplified spontaneous emissionthrough one photon or two photons and/or wherein the passivatedperovskite quantum dot has a photoluminescence quantum yield (PLQY) ofabout 70% or more.

An embodiment of the present disclosure includes, among others, a makingpassivated perovoskite quantum dot comprising: mixing a solution ofperovoskite quantum dots with a sulfur precursor solution; and forming apassivated perovskite quantum dot having a capping ligand comprised ofinorganic-organic hybrid ion pairs. In an embodiment, theinorganic-organic hybrid ion pair is a sulfur based inorganic-organichybrid ion pair.

An embodiment of the present disclosure includes, among others, a makingpassivated perovoskite quantum dot comprising: mixing a solution ofperovoskite quantum dots with a halide precursor solution; and forming apassivated perovskite quantum dot having a capping ligand comprised ofinorganic-organic hybrid ion pairs.

In some embodiments, the present invention includes an optoelectronicdevice that includes a film comprising a passivated perovskite quantumdot, where the quantum dot includes i) a core of the form APbX₃, where Ais Cs⁺, Rb⁺, CH₃NH₃ ⁺, or HC(NH₂)₂ ⁺, and X is a halogen and ii) acapping ligand that includes an inorganic-organic hybrid ion pairincluding di-dodecyl dimethylammonium and at least one anion selectedfrom the group of Cl⁻, Br⁻, I⁻, SH⁻, sulfide (S²⁻), Se²⁻, HSe⁻, Te²⁻,HTe⁻, TeS₃ ²⁻ or AsS₃ ²⁻. The optoelectronic device may be a quantumdots light emitting device (QLED) and may further comprise an electrontransport layer (e.g., a TPBi electron transport layer) and a holetransport layer (e.g., a PVK hole transportation layer).

Other compositions, methods, features, and advantages will be or becomeapparent to one with skill in the art upon examination of the followingdrawings and detailed description. It is intended that all suchadditional compositions, methods, features and advantages be includedwithin this description, be within the scope of the present disclosure,and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be more readilyappreciated upon review of the detailed description of its variousembodiments, described below, when taken in conjunction with theaccompanying drawings.

FIG. 1 is a scheme of size purification for ASE test.

FIGS. 2A-2F are TEM images (2A-2C) and corresponding size distributionsof sample 1 (2A and 2D), sample 2 (2B and 2E), and sample 3 (2C and 2F)QDs.

FIG. 3 demonstrates normalized PL emission peaks of CsPbBr₃ QDs ofvarious sizes.

FIG. 4A shows absorption and emission spectra of the untreated QD sampleand the treated QD with various amounts of sulfur; the beam width wasfixed at 1 nm for the 367 nm excitation, and FIG. 4B is thecorresponding PLQY.

FIGS. 5A-5D demonstrate energy-filtered TEM images and size distributionof the treated QDs. FIG. 5A is an energy-filtered TEM image ofsulfur-treated CsPbBr₃ QD networks. FIG. 5B is an EF-TEM image at thesame location as panel a, with lead mapped. FIG. 5C is an EF-TEM mappingof sulfur. FIG. 5D is the size distribution of QDs after treatment.

FIG. 6 shows XRD patterns of the untreated and treated samples.

FIGS. 7A-7B are photographs of treated (left) and untreated sample(right); (7A) without and (7B) under UV light irradiation.

FIG. 8 demonstrates absorption and PL spectra of CsPbBr₃ QD film beforeand after surface treatment. (Top inset) TEM image of treated QD.(Bottom inset) TEM image of untreated QD.

FIG. 9 shows FTIR spectra of untreated CsPbBr₃ QDs in toluene andtreated sample in octane. The peak at 1680 cm⁻¹ is associated with the(C═O) stretching of the original oleate ligand.

FIGS. 10A-10B are SEM images of FIG. 10A) CsPbBr₃ QDs thin film. Insetshows a high-resolution image. FIG. 10B) Cross-sectional SEM imageshowing the thickness of the film used for lasing. An ITO film was usedas a reference specimen for SEM to avoid charging effects.

FIGS. 11A-11B demonstrate the pump-fluence relationship determined forthe treated CsPbBr₃ QD film in (FIG. 11A) 1PA and (FIG. 11B) 2PA.

FIGS. 12A-12B illustrate the threshold behavior of the intensity of theASE of the treated CsPbBr₃ QD film in (FIG. 12A) 1PA and (FIG. 12B) 2PA.

FIGS. 13A-13B show threshold behavior of passivated CsPbBr₃ QDs filmbefore (FIG. 13A) and 4 months after (FIG. 13B) passivation.

FIGS. 14A-14B show shot-dependent ASE intensity of thesolution-processed CsPbBr₃ QDs film with approximately 1.2×10⁸ laserexcitation shots at (254 μJ/cm² pump fluence for 1PA (FIG. 14A), 18mJ/cm² pump fluence for 2PA (FIG. 14B)) performed at room temperatureunder ambient conditions.

FIG. 15 demonstrates time-correlated single photon counting data (TCSPC)of CsPbBr₃ QD film.

FIG. 16 shows the kinetics of nanosecond transient absorptionspectroscopy of CsPbBr₃ QD film excited at 350 nm (blue dots) and 760 nm(dark yellow dots).

FIG. 17 illustrates the multilayer perovskite QLED device structure.

FIG. 18 displays the current density and luminance versus drivingvoltage characteristics for the QLED device.

FIG. 19 shows the external quantum efficiency and luminance versusdriving voltage characteristics for the QLED device.

FIG. 20 indicates an EL spectrum at an applied voltage of 5 V and,inset, a photograph of a device

FIG. 21 displays different emission color of different perovskite QDssolution under UV light source 365 nm.

FIG. 22 is a scheme of size purification for ASE.

FIG. 23 displays absorption and PL spectra of CsPbBr₃ QD film before andafter surface treatment. The top inset is a TEM image of treated QD. Thebottom inset is a TEM image of untreated QD.

FIGS. 24A-24B illustrate the pump-fluence relationship determined forthe treated CsPbBr₃ QD film in (24A) 1PA and (24B) 2PA.

FIGS. 25A-25B illustrate the threshold behavior of the intensity of theASE of the treated CsPbBr₃ QD film in (25A) 1PA and (25B) 2PA.

FIGS. 26A-26B show shot-dependent ASE intensity of thesolution-processed CsPbBr₃ QDs film with approximately 1.2×10⁸ laserexcitation shots at (254 μJ/cm² pump fluence for 1PA, 18 mJ/cm² pumpfluence for 2PA) performed at room temperature under ambient conditions.

FIGS. 27A-27F are TEM images and corresponding size distributions ofsample 1 (27A, 27D), sample 2 (27B, 27E), and sample 3 (27C, 27F) QDs.

FIG. 28 shows normalized PL emission peaks of CsPbBr₃ QDs of varioussizes.

FIG. 29A shows absorption and emission spectra of the untreated QDsample and the treated QD with various amounts of sulfur; the beam widthwas fixed at 1 nm for the 367 nm excitation. FIG. 29B is thecorresponding PLQY.

FIG. 30A shows the energy-filtered TEM RGB image of sulfur-treatedCsPbBr₃ QD networks. FIG. 30B is the EF-TEM image at the same locationas panel 9A, with lead mapping. FIG. 30C shows EF-TEM mapping of sulfur.FIG. 30D shows the size distribution of QDs after treatment.

FIG. 31 shows XRD patterns of the untreated (bottom) and treated (top)samples.

FIGS. 32A-32B are photographs of treated (left) and untreated sample(right) (32A) without and (32B) under UV light irradiation.

FIG. 33 shows FTIR spectra of untreated CsPbBr₃ QDs in toluene andtreated sample in octane. The peak at 1680 cm⁻¹ is associated with the(C═O) stretching of the original oleate ligand.

FIGS. 34A-34B show SEM images of (FIG. 34A) CsPbBr₃ QDs thin film. Theinset shows a high-resolution image. (FIG. 34B]) Cross-sectional SEMimage is showing the thickness of the film used for lasing. An ITO filmwas used as a reference specimen for SEM to avoid charging effects.

FIGS. 35A-35B illustrate threshold behavior of passivated CsPbBr₃ QDsfilm before and 4 months after passivation.

FIG. 36 illustrates time-correlated single photon counting data (TCSPC)of CsPbBr₃ QD film.

FIG. 37 illustrates kinetics of nanosecond transient absorptionspectroscopy of CsPbBr₃ QD film excited at 350 nm (blue dots) and 760 nm(dark yellow dots).

FIGS. 38A-38F are TEM images with scale bar of 10 nm: FIG. 38A P-QDswithout wash; FIG. 38B OA-ODs, washed with butanol and dispersed intoluene; FIG. 38C OA-ODs, washed with butanol after OA soaking for 30min and dispersed in toluene;

FIG. 38D DDAB-OA-QDs, washed with butanol and dispersed in toluene; FIG.38E UV-Vis absorption and PL spectra and FIG. 38F FTIR spectra of P-QDs,OA-QDs, DDAB-OA-QDs.

FIGS. 39A-39D are the X-ray photoelectron spectroscopy studies showingthe (39A) Survey spectrum of P-QDs, (39B-39D) High resolution N 1s corelevel spectra of P-QDs, OA-QDs and DDAB-OA-QDs (washed with butanol anddispersed in toluene), respectively.

FIG. 40 displays the ligand exchange mechanism on the CsPbBr₃ QDsurface.

FIG. 41A-41D shows the thin film absorption spectra with inset showingthe Tauc plot for band gap estimations of P-QDs and DDAB-OA-QDs. FIG.41B shows photoelectron spectroscopy in air studies carried out for VBM.Inset showing the energy levels that are expressed from the vacuum,which is set at zero. FIG. 41C is a cross-sectional TEM image showingthe multiple layers. FIG. 41D is an illustration of the schematic PeLEDdevice structure.

FIGS. 42A-42F illustrate green (CsPbBr₃) PeLED device performance. FIG.42A illustrates current density and luminance versus driving voltagecharacteristics. FIG. 42B plots current efficiency and external quantumefficiency versus driving voltage characteristics. FIG. 42C shows ELspectrum at an applied voltage of 5V, and inset, a photograph of adevice. Blue (CsPbBr₃Cl_(3-x)) PeLED device performance. FIG. 42D plotscurrent density and luminance versus driving voltage characteristics.FIG. 42E plots current efficiency and external quantum efficiency versusdriving voltage characteristics. FIG. 42F shows EL spectrum at anapplied voltage of 7 V, and inset, a photograph of a device.

FIG. 43 is a series of photo images of different QDs samples. From leftto right: P-QDs, OA-QDs, and DDAB-OA-QDs, respectively.

FIG. 44 shows the X-ray diffraction pattern of P-QDs, OA-QDs, andDDAB-OA-QDs. OA-QDs and DDAB-OA-QDs were washed with butanol first andre-dispersed in toluene. For OA-QDs, the supernatant aftercentrifugation was used for characterization. However, no precipitatewas found for DDAB-OA-QDs. All of the samples were spin coated on aclean glass substrate for XRD analysis.

FIG. 45 demonstrates PL intensity stability curves as a function ofnumber of days for the P-QDs and DDAB-OA-QDs samples respectively. Boththe samples were dissolved in toluene and measured under ambientcondition.

FIG. 46 shows the XPS spectra of P-QDs for the Pb (4f), Cs (3d) and Br(3d) orbitals as marked in the figure respectively.

FIG. 47 shows the XRD pattern of the precipitate from only DDAB treatedQDs after 2.5 h.

FIG. 48 shows time-dependent PL spectra for only DDAB treated QDs.

FIGS. 49A-49D are TEM images the QDs with DDAB treatment after (FIG.49A) 2.5 h and (FIG. 49B) from supernatant; FIG. 49C and FIG. 49Dillustrate images from the precipitate. After treated 2.5 h, thetransparent solution was centrifuged at 8000 rpm for 5 min; thesupernatant was collected for directly TEM analysis while theprecipitate was re-dispersed in toluene for characterization. No regularcubic shape around 10 nm can be found, indicated possible new reactioninside the mixture.

FIGS. 50A-50B are SEM images showing the surface morphology ofspin-coated thin films of (FIG. 50A) P-QDs and (FIG. 50B) DDAB-OA-QDs.

FIGS. 51A-51B illustrate performance of control device using P-QDs asemitting layer. FIG. 51A plots external quantum efficiency and luminanceversus driving voltage. FIG. 51B plots current density and luminanceversus driving voltage.

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, and as such may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the present disclosure will be limited onlyby the appended claims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range, is encompassed within the disclosure. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges and are also encompassed within the disclosure, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present disclosure, the preferredmethods and materials are now described.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentdisclosure. Any recited method can be carried out in the order of eventsrecited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwiseindicated, techniques of chemistry, material science, and the like,which are within the skill of the art.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toperform the methods and use the probes disclosed and claimed herein.Efforts have been made to ensure accuracy with respect to numbers (e.g.,amounts, temperature, etc.), but some errors and deviations should beaccounted for. Unless indicated otherwise, parts are parts by weight,temperature is in ° C., and pressure is at or near atmospheric. Standardtemperature and pressure are defined as 20° C. and 1 atmosphere.

Before the embodiments of the present disclosure are described indetail, it is to be understood that, unless otherwise indicated, thepresent disclosure is not limited to particular materials, reagents,reaction materials, manufacturing processes, or the like, as such canvary. It is also to be understood that the terminology used herein isfor purposes of describing particular embodiments only, and is notintended to be limiting. It is also possible in the present disclosurethat steps can be executed in different sequence where this is logicallypossible.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise.

Definitions

The term “quantum dot” can include, but is not limited to, luminescentsemiconductor quantum dots. In general, quantum dots include a core andoptionally a cap. The “core” is a nanometer-sized semiconductor. Whileany core of the IIA-VIA, IIIA-VA, or IVA-VIA semiconductors can be usedin the context of the present disclosure, the core is such that, uponcombination with a cap, a luminescent quantum dot results. A IIA-VIAsemiconductor is a compound that contains at least one element fromGroup IIA and at least one element from Group VIA of the periodic table,and so on. The core can include two or more elements. In one embodiment,the core is a IIA-VIA, or IIIA-VA, semiconductor that can be about 1 nmto about 250 nm, about 1 nm to 100 nm, about 1 nm to 50 nm, or about 1nm to 10 nm in diameter. In another embodiment, the core can be aIIA-VIA semiconductor and can be about 2 nm to about 10 nm in diameter.For example, the core can be CsPbBr₃, CdS, CdSe, CdTe, ZnSe, ZnS, PbS,PbSe, CsPbCl₃, CsPbCl_(3-x)Br_(x), (x is 0 to 3), or an alloy.

The wavelength emitted (e.g., color) by the quantum dots can be selectedaccording to the physical properties of the quantum dots, such as thesize and the material of the nanocrystal. Quantum dots are known to emitlight from about 300 nanometers (nm) to 2000 nm (e.g., UV, near IR, andIR). The colors of the quantum dots include, but are not limited to,red, blue, green, and combinations thereof. The color or thefluorescence emission wavelength can be tuned continuously. Thewavelength band of light emitted by the quantum dot is determined byeither the size of the core or the size of the core and cap, dependingon the materials that make up the core and cap. The emission wavelengthband can be tuned by varying the composition and the size of the QDand/or adding one or more caps around the core in the form of concentricshells.

The phrase “amplified spontaneous emission” means light, produced byspontaneous emission, that has been optically amplified by the processof stimulated emission in a gain medium. It is inherent in the field ofrandom lasers.

Two-photon absorption, or 2PA, as used herein is a third-ordernon-linear optical phenomenon in which a molecule absorbs two photonssimultaneously, resulting in an electronic transition from the groundstate to an excited state via virtual states. 2PA pumping has severaladvantages over 1PA pumping, for instance, minimum risk of photodamageto the sample, longer penetration depth in the absorbing material andthe absence of a phase-matching requirement for the generation andwavelength tuning of coherent light. In addition to these advantages,deleterious effects of the excitation light such as unwanted scatteringand absorption losses are completely suppressed in 2PA.

General Discussion

Embodiments of the present disclosure provide for passivated quantumdots, methods of making passivated quantum dots, methods of usingpassivated quantum dots, and the like. In an embodiment, the passivatedquantum dots can be used to make structures such as quantum films aswell as be used in non-linear optical applications, solar cells, LEDs,photovoltaics, lasing, photodetectors and other optoelectronicapplications. Embodiments of the present disclosure provide forpassivated quantum dots and films that exhibit high pump fluence andoperational stability in ambient conditions as compared to currentlyused technologies. Furthermore, embodiments of the present disclosureare advantageous over other perovskite materials due to their increasedphotostability under laser excitation for both one- and two-photonpumping.

In particular, the present disclosure provides for ultra-air stableand/or photostable passivated quantum dots (e.g., CsPbBr₃ Quantum Dots(QDs)) that include an inorganic-organic hybrid ion pair as the cappingligand for the core of the QD. This passivation approach to perovskiteQDs yields high photoluminescence quantum yield with unprecedentedoperational stability in ambient conditions (e.g., about 60±5% labhumidity) and high pump fluences, thus overcoming one of the greatestchallenges impeding the development of perovskite-based applications.Due to the robustness of passivated perovskite QDs, induced ultra-stableamplified spontaneous emission (ASE) in solution of processed QD filmscan be conducted using one photon as well as through two-photonabsorption processes. The two photon process has not been observedbefore in the family of perovskite materials. In addition, passivatedperovskite QD films showed remarkable photostability under continuouspulsed laser excitation in ambient conditions for at least 34 hours(corresponds to about 1.2×10⁸ laser shots), substantially exceeding thestability of other colloidal QD systems in which ASE has been observed.

In an embodiment, the composition includes a passivated perovskitequantum dot. In an embodiment, the core of the passivated quantum dotcan be a IIA-VIA, IIIA-VA, or IVA-VIA semiconductors such that uponcombination with a capping ligand (cap) forms a luminescent quantum dot.In an embodiment, the passivated perovskite quantum dot includes a coreof the form APbX₃, where A is Cs⁺, Rb⁺, CH₃NH₃ ⁺, or HC(NH₂)₂ ⁺, and Xis a halogen. In an embodiment, the capping ligand of the passivatedperovskite quantum dot is an inorganic-organic hybrid ion pair. In anembodiment, the inorganic-organic hybrid ion pair is a sulfur basedinorganic-organic hybrid ion pair or a halide based inorganic-organichybrid ion pair. The sulfur based inorganic-organic hybrid ion pairinclude S²⁻ and di-dodecyl dimethylammonium (DDA⁺). In anotherembodiment, the halide inorganic-organic hybrid ion pair can include adi-dodecyl dimethylammonium bromide (Br⁻-DDA⁺), di-dodecyldimethylammonium chloride (Cl⁻-DDA⁺), and a combination thereof. In anembodiment, the other anion could also be applied for theinorganic-organic hybrid ion pair, such as Cl⁻, Br⁻, I⁻, SH⁻, Se²⁻,HSe⁻, Te²⁻, HTe⁻, TeS₃ ²⁻ or AsS₃ ²⁻, while the other can be di-dodecyldimethylammonium (DDA⁺).

In an embodiment, the size (e.g., diameter) of the quantum dot can beabout 5 to 20 nm or about 6 to 16 nm. In an embodiment, the thickness ofthe capping ligand layer can be about 70 to 140 nm or about 90 to 120nm. In an embodiment the diameter of the passivated perovskite quantumdot can be about 75 to 160 nm or about 75 to 150 nm.

In an embodiment, the passivated perovskite quantum dot can have acharacteristic of being able to have an amplified spontaneous emissionthrough one photon or two photons. In some embodiments, the amplifiedspontaneous emission (ASE) of the passivated quantum films through one-and two-photon pumping can be about 500 to 540 nm, and thephotoluminescent emission spectrum can about 3 to 10 nm over thethreshold pumping range.

In addition, the passivated perovskite quantum dot has aphotoluminescence quantum yield (PLQY) of about 50% or more (e.g., about70 to 99%). In certain embodiments, the photoluminescent quantum yieldof the passivated quantum dots can be about 50-80%.

In an embodiment, the passivated perovskite quantum dot has anoperationally stability in ambient conditions of about 55 to 65% labhumidity. In an embodiment, the passivated perovskite quantum dot has ahigh pump fluence (e.g., about 190 μJ/cm²).

As mentioned above, the passivated QDs can be incorporated into apassivated QD film. In an embodiment, the passivated perovskite QD filmhas a photostability under continuous pulsed laser excitation in ambientconditions for at least about 34 hours or more (e.g., about 34 hourscorresponds to 1.2×10⁸ laser shots).

In an embodiment, the passivated QDs were applied in a greenlight-emitting diode (LED) application with an inverted devicearchitecture: ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al (FIG. 17 of Example 1),achieved luminance and external quantum efficiencies of 330 cd m⁻² and3%, respectively (FIGS. 18, 19 of Example 1), shown anelectroluminescence of 515 nm (FIG. 20). Based on the promising result,the full color gamut in the visible range can be realized via a simplehalide anion exchange (FIG. 21 of Example 1). Similar blue, red and evenwhite LEDs can be produced.

An embodiment of the present disclosure includes methods of makingpassivated perovoskite quantum dots. A solution of perovoskite quantumdots is mixed with a precursor solution (e.g., a sulfur precursorsolution or a halide precursor solution). A method of formingperovoskite quantum dot cores is provided in the Example and can beapplied to forming different types of perovoskite quantum dot cores. Amethod of making the sulfur solution is provided in the Example and canbe applied to forming different types of sulfur solutions such asdi-dodecyl dimethylammonium sulfide (S²⁻-DDA). In another embodiment,the halide precursor solution can include a di-dodecyl dimethylammoniumbromide (Br⁻-DDA⁺), di-dodecyl dimethylammonium chloride (Cl⁻-DDA⁺), anda combination thereof. Subsequently, the passivated perovskite quantumdots having the capping ligand are formed.

For example, an acid (e.g., oleic acid) is mixed with the perovoskitequantum dots to form a first mixture. The first mixture is mixed withthe precursor solution (e.g., a sulfur precursor solution or a halideprecursor solution), and the reaction product is precipitated and thenre-dissolved in a solvent. The passivated perovskite quantum dots can bethen be processed as desired. Additional details are provided in theExamples.

EXAMPLES Example 1

Lead trihalide perovskite materials with the formula APbX₃ (where A=Cs⁺,CH₃NH³⁺, or HC(NH₂)²⁺; X=Br⁻, I⁻, and/or Cl⁻) have recently emerged aspromising materials for solution-processed optoelectronic applicationssuch as photovoltaics,¹⁻⁶ lasing,⁷⁻¹⁰ light-emitting diodes¹¹⁻¹⁴ andphotodetectors¹⁵⁻¹⁷ because of their easily tunable optical bandgap aswell as their attractive absorption, emission, and charge transportproperties. APbX₃ materials with some, or all, of these characteristics,have been realized in various forms, including thin films,¹⁸ singlecrystals,¹⁹⁻²⁰ nanowires,²¹⁻²² and quantum dots.²³⁻²⁴ Perovskite QDs inparticular, such as CsPbX₃, have ushered the advantages of perovskitematerials into the realm of quantum confinement. The results are QDswith emission lines that exhibit narrow full widths at half maximum(FWHM) and remarkably high photoluminescence quantum yields (PLQYs)(approximately ≥70%) and yet can be easily tuned over a very broadspectral window²⁵

Despite the impressive metrics of the entire family of lead trihalideperovskites, however, poor stability, in particular with respect totemperature, moisture, and light exposure, remains a ubiquitousimpediment for virtually all APbX₃ perovskite materials anddevices.^(26,27) The lack of stability has not only prevented thepractical and commercial application of perovskite optoelectronics buthas also constrained the exploration of their properties to highlysanitized conditions.²⁸ Under such a restrictive regime, non-linearoptical properties essential for lithography²⁹, high-resolution opticalmicroscopy,³⁰ and the characterization and generation of ultrafastoptical signals³¹ suffer the most. For the aforementioned reasons,surface passivation strategies of perovskite materials have beenvigorously pursued (for example, Snaith et. al employed various organicLewis bases such as thiophene and pyridine to passivate the surface ofperovskite films to achieve high photovoltaic power conversionefficiency),³² but to date, none have led to operational and long-termstability under ambient conditions and/or at high optical fluence.

Herein, we report the development of ultra-air- and photostableperovskite QDs through a new passivation technique (FIG. 1) in which thesurface of QDs is coated with an inorganic-organic hybrid ion pair.³³This novel approach yields high photoluminescence quantum yield (PLQY)with remarkably high operational stability in ambient conditions (60±5%lab humidity) and high pump fluences, thus overcoming one of the majorchallenges impeding the development of perovskite-based applications. Byachieving such levels of robustness, we were able to induce with nosigns of degradation amplified spontaneous emission (ASE) insolution-processed QD films not only through one photon (1PA) but alsothrough two-photon absorption (2PA) processes. ASE with the latter inparticular has not been previously reported before in the family ofperovskite materials, and it demonstrates the potential of thesematerials for non-linear optical applications.

CsPbBr₃ quantum dots (QDs) were synthesized similarly to the modifiedhot-injection method previously reported by Protesescu et al.²⁵ As shownin FIG. 1, selective centrifugation of the crude solution yielded threesamples of various QD sizes. Size distribution analysis revealed thepresence of three different populations of QDs (referred to as samples1, 2 and 3) with average sizes of ˜8.2 nm, 9.2 nm, and ˜10.6 nm,respectively (FIGS. 2A-2B). To investigate the quantum size effect, wemeasured the photoluminescence spectra of the synthesized samples. Clearred spectral shifts of 9 nm and 16 nm were observed in the PL positionof samples 2 and 3, relative to the spectrum of sample 1, which isconsistent with quantum confinement (FIG. 3). This observation is inline with recent reports on the same type of QDs.²⁴⁻²⁵

The passivation of surface defects in nanosized organometal halideperovskites can lead to a substantial increase in the photoluminescencequantum yield (PLQY) at room temperature³⁴; however, the air andphotostabilities of these materials remained elusive. The current studyaimed to improve both the PLQY and stability by passivating the QDs, aprerequisite for using them in optoelectronic applications. Initially,we observed that the synthesized CsPbBr₃ QDs displayed a lower PLQYcompared to previously reported values.²⁵ Moreover, the PLQY depended onthe size of the QDs, reaching values of approximately 35%, 49% and 49%for samples 1, 2 and 3, respectively. To overcome these critical issues,we introduced an inorganic-organic hybrid ion pair (di-dodecyldimethylammonium sulfide, S²⁻-DDA₊)³⁵ to passivate the QDs, di-dodecyldimethyl ammonium bromide (DDAB) was used as a source of DDA₊ topassivate the perovskite CsPbBr₃ QDs. While inorganic-organic hybrid ionpairs, such as AsS₃ ³⁻-DDA⁺, where used to cap metal chalcogenidenanocrysta^(I33), their implementation for perovskite materials has notbeen demonstrated. In this method, 50 μl of oleic acid (OA) was added to1 mL of CsPbBr₃ QDs (15 mg/mL) under stirring, followed by the additionof a certain amount of sulfur precursor (see Experimental Section fordetails). The reaction product was subsequently precipitated with BuOHand re-dissolved in octane. We observed that the solution PLQY wasenhanced from 49% to 70% upon the injection of 100 μl of sulfurprecursor (FIG. 4B). Notably the original position of the emission peakof the CsPbBr3 QDs remained almost unaltered upon the addition of thesulfur precursor, and only an enhancement was observed (FIG. 4A). Tounderstand the nature and the composition of the surface passivation, weemployed energy-filtered TEM (EFTEM) analysis, from which we observedthat sulfur plays a role in passivation (FIGS. 5A-5D). Also, CHNSelemental analysis further corroborated the existence of sulfur upontreatment (Table 1). Furthermore, the calculated elemental ratio of C,H, and N support the formation of hybrid ion pair (S²⁻-DDA⁺).

EF-TEM characterization was conducted on sulfur-treated nanocrystals.Elemental mapping was carried out for lead and sulfur. Results indicatea high correlation between lead and sulfur (FIGS. 5B-5D), suggestingthat sulfur was indeed present on the nanocrystals as a capping layerrather than as a PbS shell.

TABLE 1 Elemental analysis results Sample sample weight (mg) N (%) C (%)H (%) S (%) CsPbBr₃ 3.766 0.54 8.16 1.35 0 Treated 5.262 0.30 6.26 1.110.20 CsPbBr₃

Passivated QD samples were characterized by X-ray powder diffraction(XRD). The XRD patterns of the treated samples were well indexed to thestandard cubic CsPbBr3 phase, with no observable secondary phases (FIG.6). On the other hand, untreated samples exhibited a diffraction patternthat cannot be indexed clearly to the pristine cubic perovskite phase,likely as a result of the samples' degradation under ambient conditions.The instability and degradation of an untreated QD sample were alsosubstantiated by X-ray fluorescence (XRF) analysis, as indicated by the1:2:5 atomic ratios of Cs, Pb, and Br, respectively, a considerabledeviation from the 1:1:3 ratio expected for perovskites (Table 2).Moreover, treated samples were noticeably brighter than the untreatedsamples under UV light, a clear indicator of the high luminescenceobtained after passivation (FIGS. 7A-7B).

TABLE 2 X-ray fluorescence (XRF) results of untreated sample Elem. LineMass[%] 3 sigma Atomic[%] 35 Br K 42.93 0.28 63.06 55 Cs K 14.54 0.4212.84 82 Pb L 42.53 0.3 24.1

Both the treated and untreated samples were washed with 1-butanol,centrifuged and placed under vacuum. The color varied and theluminescence was improved with the applied treatment. X-ray fluorescence(XRF) was used to analyze the major elements of the sample. The Cs:Pb:Brratio was approximately 1:2:5 for the untreated sample (see Table 2),confirming the instability of the inorganic perovskite, which isconsistent with the XRD pattern (FIG. 6). The elemental ratio of thetreated sample obtained by XRF analysis is not reasonable, perhapsbecause the sulfur K line (2.307 keV) is very close to the lead M line(2.42 keV) and because hybrid ion pair ligand passivation was performed.However, the XRD pattern (FIG. 6) of the treated sample corresponds tocubic phase CsPbBr₃, indicating high stability in air (60% humidity inthe lab).

For optical characterizations, the untreated and treated samples werespin-coated on a glass substrate to obtain a uniform thin film (seeExperimental Section). FIG. 8 shows the absorption and photoluminescence(PL) spectra of the QD films before and after surface passivation. Itshould be noted that the PL signal is quite narrow in both cases, withan FWHM of 25 nm. Only a very slight change in the bandgap could beobserved, as shown in the Tauc plot (FIG. 8, inset), suggesting that theparticle size was essentially preserved even after the surfacetreatment. In addition to the characterizations performed by electronicspectroscopy, vibrational spectroscopy was used to probe the specificchemical functionalities on the surface of the QDs. The C═O stretchingvibration of OA was used as a specific marker mode to probe the ligandexchange process. In this experiment, the absence of the 1680 cm⁻¹ peak(C═O stretching) in the FTIR spectrum of the treated sample (FIG. 9)provided a clear indication of the replacement of the native oleatefunctional group by the hybrid ion pair.

To confirm the air and photostability of the passivated CsPbBr₃ QDsfilms (thickness ˜105 nm, FIGS. 10A-10B), we tested their propensity foramplified spontaneous emission (ASE) through one- and two-photon pumpingunder ambient conditions (room temperature, 60±5% humidity). The resultsare shown in FIGS. 11A-11B. As can be seen, with an increase in the pumpfluence, the ASE generated at 533 nm shifted to 537 nm, which isconsistent with the results of a previous report.³⁶ The onset ofstimulated emission in the QD films was observed with the immediateincrease in the ASE intensity (ASE peak at ≈, 533 nm) and the narrowingof the emission spectrum (FWHM≈4-7 nm, FIG. 11A inset) over thethreshold pumping range. A red shift in the ASE peak (16-21 nm) relativeto the PL peak was observed (FIG. 11B).

As shown in FIG. 12A, the ASE threshold fluence for one-photon pumpingwas approximately 192 μJ/cm², which is lower than the value reported forperovskite thin films.³⁷ On the other hand, the threshold obtained forthe QD film in the current study is higher than that recently reportedfor the same QDs,³⁶ probably due to differences in the configurations ofthe experimental setups. However, the air and photostabilities of ourtreated QDs are the best reported to date for semiconductor QDs and evenfor perovskite thin films. It is worth noting that the untreated samplesuffered from partial degradation within hours, showing a sharp contrastin performance, whereas the passivated QD film exhibited the samethreshold and optical characteristics even after 4 months of open airstorage and rounds of photostability testing (vide infra and also seeFIG. 13A-13B), providing clear evidence of the ultra-stability ofperovskite QD films under ambient conditions.

To test the photostability of our QDs films, we measured the variationin ASE intensity as a function of time under continuous femtosecondlaser irradiation in ambient conditions, using a femtosecond lasersystem operated at a repetition rate of 1 kHz. Our results showed thatthere was almost no change in ASE intensity over 1.2×10⁸ laser shots(corresponding to a period of 34 hours) for either one- or two-photonpumping (FIGS. 14A-14B). This value substantially exceeds thephoto-stability of other semiconductor QD systems for which ASE has beenobserved.^(38,39)

We studied the excited-state dynamics of QDs by the time-correlatedsingle photon counting (TCSPC) technique. At pump fluences below the ASEthreshold (8.5 μJ/cm², FIG. 15), typical PL lifetime of 11 ns wasobserved. Well above the ASE threshold (220 μJ/cm²; FIG. 15), an ASElifetime of approximately 2.8 ns was recorded, which further decreasedto 1.9 ns with the increase in pump fluence. Such a fast decay withincreasing pump fluence could be attributed to the dominant contributionfrom the non-radiative decay channel.

We further investigated the pumping of the passivated perovskite QDswith a non-linear 2PA scheme. Note that 2PA is a third-order non-linearoptical phenomenon in which a molecule absorbs two photonssimultaneously, resulting in an electronic transition from the groundstate to an excited state via virtual states. 2PA pumping has severaladvantages over 1PA pumping, for instance, minimum risk of photodamageto the sample, longer penetration depth in the absorbing material andthe absence of a phase-matching requirement for the generation andwavelength tuning of coherent light.³⁹ In addition to these advantages,deleterious effects of the excitation light such as unwanted scatteringand absorption losses are completely suppressed in 2PA.

The ASE-induced in the perovskite QDs during optical pumping wasachieved via 2PA at 800 nm. The following wavelength range highlightsthe distinct advantages offered by 2PA pumping for the perovskite QDsbecause high-powered laser sources are abundant within this range;moreover, the range is also an optical transparency window for water andbiological media. It is worth noting that below and above the thresholdfluence, the FWHM and peak position achieved through 2PA pumping areindistinguishable from those achieved by 1PA excitation; however, adecrease in the intensity of the ASE achieved by 2PA was observed.Femtosecond transient absorption with broadband capabilities shows thatthe dynamics of 2PA-induced ground-state bleaching were similar to thoseof 1PA, and the recovery time fits a single exponential function with acharacteristic time constant of ˜20 ns (FIG. 16).

Although the excited-state dynamics were observed to be identical, 1PAand 2PA ASE showed markedly different thresholds. For two-photonexcitation, the ASE threshold fluence was 12 mJ/cm² (FIG. 12B), which iscomparable to that reported for 2PA ASE in other semiconductor QDs.⁴⁰Nevertheless, the stability of our passivated perovskite QDs under theapplied operating conditions largely surpasses (photostability unchangedafter 1.2×10⁸ laser shots) all previously reported levels for ASE inQDs, regardless of whether 1PA or 2PA induced that emission. Althoughfew reports on the 2PA pumping of colloidal QDs exist^(39, 41-43) ASEfrom 2PA pumping in the perovskite family of materials has not beenreported prior to this work.

In summary, a passivation strategy for lead halide perovskite QDs wasdeveloped to alleviate the inherent instability of the material whenoperating under ambient conditions currently is the greatest challengeimpeding the development of perovskite-based devices. Our passivationstrategy is endowed perovskite QDs with unprecedented stability in theair (60±5% humidity) and under high laser fluences, with samples showingno noticeable degradation. An analytical investigation of the passivatedperovskite QDs revealed the formation of a protective layer enrichedwith sulfide (S²⁻-DDA⁺). Because of the ultra-high stability of theresulting QDs, we were able to induce ultra-stable ASE insolution-processed QD films not only by 1PA but also by a 2PA process.ASE in the latter is a phenomenon that has yet to be observed in anyperovskite material. Additionally, our perovskite QD films showedsignificant photo-stability under continuous pulsed laser excitation inambient conditions for at least 34 hours (corresponds to 1.2×10⁸ lasershots), substantially exceeding the stability of other colloidal QDsystems in which ASE has been observed. The described QD passivationstrategy and multiphoton-induced processes enable the practicalimplementation of perovskite QDs and facilitate the exploration of boththeir linear and non-linear applications. We believe that this surfacepassivation mechanism and the validity of the 2PA process will open newavenues of study and hold promise for overcoming the greatest problemprecluding the development of perovskite-based materials for solar celland non-linear applications.

EXPERIMENTAL SECTION

Synthesis

1-butanol (BuOH, HPLC grade), was purchased from Fisher Scientific.Oleic acid (OA, technical grade 90%), lead bromide (PbBr₂, 98%) andoctane (98%) were purchased from Alpha Aesar. Sodium sulfide hydrate,cesium carbonate (Cs₂CO₃, 99.995%, metal basis), di-dodecyl dimethylammonium bromide (DDAB, 98%), oleylamine (OLA, technical grade 70%), and1-octadecene (ODE, technical grade 90%) were purchased fromSigma-Aldrich. Toluene (HPLC grade) was purchased from Honeywell Burdick& Jackson. All chemicals were used as procured without furtherpurification.

Preparation of Cs-Oleate

Cs₂CO₃ (0.814 g) along with ODE (40 mL) and OA (2.5 mL) were loaded intoa 100 mL two-neck flask, dried for 1 h at 120° C., and then heated underN2 at 150 QC until all Cs2CO3 had reacted with OA. The solution wasmaintained at 150 QC before injection to prevent the solidification ofCs-oleate.

Preparation of S Precursor (S²⁻-DDA⁺)

Similarly to Jiang's method,³⁵ 3 mL of toluene containing 0.15 m mol ofDDAB was mixed with 3 mL of 50 mM aqueous Na₂S solution. The S²⁻ anionswere then transferred from the aqueous phase to the toluene phase. Thetoluene phase was separated and used as a sulfur precursor (DDA-S²⁻) insubsequent experiments.

Synthesis of CsPbBr₃ QDs

CsPbBr₃ quantum dots (QDs) were synthesized via a modified hot-injectionmethod. ODE (125 mL), OLA (12.5 mL), OA (12.5 mL) and PbBr2 (1.725 g)were loaded into a 500 mL two-neck round-bottom flask and dried undervacuum for 1 h at 120° C. After the PbBr2 salt had completely dissolved,the temperature was raised to 180° C. under N2 gas. 10 mL Cs-oleatesolution (0.814 g Cs2CO3, 40 ml ODE and 2.5 ml OA were loaded into a 100mL two-neck flask, dried under vacuum for 1 h at 120° C., and thenheated under N2 at 150° C. The solution was maintained at 150° C. beforeinjection to prevent the solidification) was quickly injected; anice-water bath was then used to cool the reaction mixture after 5seconds. The crude solution was purified by selective centrifugation asshown in FIG. 1, yielded three samples with average sizes of ˜8.2 nm,9.2 nm and ˜10.6 nm, respectively (referred to as samples 1, 2 and 3).To investigate the quantum size effect, we measured thephotoluminescence spectra of the synthesized samples. Clear spectral redshifts of 9 nm and 16 nm were observed in the PL position of samples 2and 3 relative to the spectrum of sample 1, which is consistent withquantum confinement (FIGS. 2A-2F).

Selected Purification of CsPbBr₃ QDs

The crude solution was cooled in a water bath and directly transferredto centrifuge tubes. After centrifuging the tubes at 7000 rpm for 15min, the supernatant and precipitate were collected separately. Thesupernatant was mixed with BuOH for centrifugation, and the bottomsample was then collected and re-dissolved in toluene (sample 1). Theprecipitate was dispersed by adding toluene to collect the newsupernatant (sample 2) after centrifugation and re-disperse the newprecipitate in toluene (sample 3).

Treatment of CsPbBr₃ QDs and Preparation of CsPbB_(r3) QDs Films

To 1 mL of the different CsPbBr₃ QDs (15 mg/mL), 50 μL of OA was addedunder stirring. A certain amount of sulfur precursor was then addedsequentially. The sample was precipitated with twice the amount of BuOHand re-dissolved in 200 μL of octane. Thin films of CsPbBr₃ QDs wereobtained by spin-coating the treated CsPbBr₃ QD solution under ambientconditions onto glass substrates. Moreover, the untreated sample waswashed with BuOH alone and re-dissolved in octane to fabricate the thinfilms. The glass substrates were cleaned by standard procedure withdetergent, de-ionized water, acetone, and isopropanol. The cleanedsubstrates were treated with plasma for 5 min before depositing the QDfilms. Highly smooth, densely packed and pinhole-free thin films wereobtained by spin-coating at 500 rpm (10 sec) and then at 1500 rpm (40sec).

Characterization

UV-vis absorption spectra were obtained using an absorptionspectrophotometer from Ocean Optics. Carbon, hydrogen, oxygen, andsulfur analysis was performed using a Flash 2000 elemental analyzer(Thermo Fischer Scientific). Photoluminescence was tested using anFLS920 dedicated fluorescence spectrometer from Edinburgh Instruments.Quantum yield was measured using an Edinburgh Instruments integratingsphere with an FLS920-s fluorescence spectrometer. FTIR was performedusing a Nicolet 6700 FT-IR spectrometer. Powder X-ray diffraction (XRD)patterns were recorded using Siemens diffractometer with Cu Kα radiation(λ=1.54178 Å). TEM analysis was carried out with a Titan™ TEM (FEICompany) operating at a beam energy of 300 keV and equipped with aTridiem™ post-column energy filter (Gatan, IQD.). The samples wereimaged in energy-filtered TEM (EFTEM) mode with a 20 eV energy slitinserted around the zero-energy-loss electrons to acquirehigh-resolution TEM (HRTEM) micrographs. The spatial distribution of theelements Pb and S was determined and acquired using the EFTEM techniqueby selecting the Pb 0-edge (86 eV) and S L-edge (165 eV) in thethree-window mapping method. Morphological investigations andcross-sectional imaging of the QD films were carried out on a Karl ZeissFESEM.

Experimental Details of Optical Pumping, Single Photon Counting andTransient Absorption

All ASE pumping experiments were conducted at room temperature. The 1PApumping experiments were performed using a femtosecond laser systemoperated at a wavelength 800 nm with 35 fs pulses and a repetition rateof 1 kHz. UV pump pulses at 400 nm were obtained in a straightforwardmanner by the second harmonic (frequency doubled) of the fundamentalbeam, where 100 μJ of the laser output was focused in a 100 μm BBOnonlinear crystal. The 2PA pumping experiments were carried out bydirectly using the fundamental beam at 800 nm.

Time-correlated single photon counting (TCSPC) for lifetime measurementswas performed using a Halcyone MC multichannel fluorescenceup-conversion spectrometer (Ultrafast Systems) at an excitationwavelength of 400 nm. Nanosecond transient absorption experiments wereconducted using an EOS setup (Ultrafast Systems). Detailed informationregarding the experimental setup has been published elsewhere.⁴⁴⁻⁴⁵

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Example 2

We demonstrate ultra-air- and photostable CsPbBr₃ quantum dots (QDs) byusing an inorganic-organic hybrid ion pair as the capping ligand. Thispassivation approach to perovskite QDs yields high photoluminescencequantum yield with unprecedented operational stability in ambientconditions (60±5% lab humidity) and high pump fluences, thus overcomingone of the greatest challenges impeding the development ofperovskite-based applications. Due to the robustness of passivatedperovskite QDs, we were able to induce ultrastable amplified spontaneousemission (ASE) in solution processed QD films not only through onephoton but also through two-photon absorption processes. The latter hasnot been observed before in the family of perovskite materials. Moreimportantly, passivated perovskite QD films showed remarkablephotostability under continuous pulsed laser excitation in ambientconditions for at least 34 h (corresponds to 1.2×10⁸ laser shots),substantially exceeding the stability of other colloidal QD systems inwhich ASE has been observed.

Lead trihalide perovskite materials with the formula APbX₃ (where A=Cs⁺,CH₃NH³⁺, or HC(NH₂)²⁺; X=Br⁻, I⁻, and/or Cl⁻) have recently emerged aspromising materials for solution-processed optoelectronic applicationssuch as photo-voltaics,¹⁻⁶ lasing,⁷⁻¹⁰ light-emitting diodes,¹¹⁻¹⁴ andphoto-detectors¹⁵⁻¹⁷ because of their easily tunable optical bandgap aswell as their attractive absorption, emission, and charge transportproperties.¹⁸ APbX₃ materials¹⁹ with some, or all, of thesecharacteristics, have been realized in various forms, including thinfilms,²⁰ single crystals,^(21,22) nanowires, ^(23,24) and quantumdots.^(25,26) Perovskite quantum dots (QDs) in particular, such asCsPbX₃, have ushered the advantages of perovskite materials into therealm of quantum confinement. The results are QDs with emission linesthat exhibit narrow widths at half-maximum (fwhm) and remarkably highphoto-luminescence quantum yields (PLQYs) (approximately ≥70%) and yetcan be easily tuned over a very broad spectral window.²⁷⁻³⁰

Despite the impressive metrics of the entire family of lead trihalideperovskites,³¹ however, poor stability, in particular with respect totemperature, moisture, and light exposure, remains a ubiquitousimpediment for virtually all APbX₃ perovskite materials anddevices.^(32,33) The lack of stability has not only prevented thepractical and commercial application of perovskite optoelectronics buthas also constrained the exploration of their properties to highlysanitized conditions. ³⁴ Under such a restrictive regime, nonlinearoptical properties essential for lithography, ³⁵ high-resolution opticalmicroscopy, ³⁶ and the characterization and generation of ultrafastoptical signals³⁷ suffer the most. For the aforementioned reasons,surface passivation strategies of perovskite materials have beenvigorously pursued (for example, Snaith et. al employed various organicLewis bases such as thiophene and pyridine to passivate the surface ofperovskite films to achieve high photovoltaic power conversionefficiency), ³⁸ but to date, none have led to operational and long-termstability under ambient conditions and/or at high optical fluence.

Herein, this example discusses the development of ultra-air- andphotostable perovskite QDs through a new passivation technique (FIG. 22)in which the surface of QDs is coated with an inorganic-organic hybridion pair. ³⁹ This novel approach yields high PLQY with remarkably highoperational stability in ambient conditions (60±5% lab humidity) andhigh pump fluences, thus overcoming one of the major challenges impedingthe development of perovskite-based applications. By achieving suchlevels of robustness, we were able to induce with no signs ofdegradation amplified spontaneous emission (ASE) in solution-processedQD films not only through one photon (1PA) but also through two-photonabsorption (2PA) processes. ASE with the latter in particular has notbeen previously reported before in the family of perovskite materials,and it demonstrates the potential of these materials for nonlinearoptical applications. CsPbBr₃ quantum dots (QDs) were synthesizedsimilarly to the modified hot-injection method previously reported byProtesescu et al.²⁷ As shown in FIG. 22, selective centrifugation of thecrude solution yielded three samples of various QD sizes. Sizedistribution analysis revealed the presence of three differentpopulations of QDs (referred to as samples 1, 2, and 3) with averagesizes of ˜8.2 nm, 9.2 nm, and ˜10.6 nm, respectively (FIGS. 27A-27F). Toinvestigate the quantum size effect, we measured the photoluminescencespectra of the synthesized samples. Clear red spectral shifts of 9 and16 nm were observed in the PL position of samples 2 and 3, relative tothe spectrum of sample 1, which is consistent with quantum confinement(FIG. 28). This observation is in line with recent reports on the sametype of QDs.^(26,27)

The passivation of surface defects in nanosized organometal halideperovskites can lead to a substantial increase in the photoluminescencequantum yield (PLQY) at room temperature;⁴⁰ however, the air andphotostabilities of these materials remained elusive. The current studyaimed to improve both the PLQY and stability by passivating the QDs, aprerequisite for using them in optoelectronic applications. Initially,we observed that the synthesized CsPbBr₃ QDs displayed a lower PLQYcompared to previously reported values.²⁷ Moreover, the PLQY depended onthe size of the QDs, reaching values of approximately 35%, 49%, and 49%for samples 1, 2 and 3, respectively. To overcome these critical issues,we introduced an inorganic-organic hybrid ion pair (didodecyldimethylammonium sulfide, S²⁻-DDA⁺)⁴¹ to passivate the QDs, didodecyldimethylammonium bromide (DDAB) was used as a source of DDA⁺ (see theExperimental Section in the sulfur precursor part) to passivate theperovskite CsPbBr₃ QDs. While inorganic-organic hybrid ion pairs, suchas AsS₃ ³⁻-DDA⁺, were used to cap metal chalcogenide nanocrystal,³⁹their implementation for perovskite materials has not been demonstrated.In this method, 50 μL of oleic acid (OA) was added to 1 mL of CsPbBr₃QDs (15 mg/mL) under stirring, followed by the addition of a certainamount of sulfur precursor (see Experimental Section for details). Thereaction product was subsequently precipitated with BuOH and redissolvedin octane. We observed that the solution PLQY was enhanced from 49% to70% upon the injection of 100 μL of sulfur precursor (FIG. 29B). Notablythe original position of the emission peak of the CsPbBr₃ QDs remainedalmost unaltered upon the addition of the sulfur precursor, and only anenhancement was observed (FIG. 29A). To understand the nature and thecomposition of the surface passivation, we employed energy-filteredtransmission electron microscopy (EFTEM) analysis, from which weobserved that sulfur plays a role in passivation (FIG. 30A-30C). Also,CHNS elemental analysis further corroborated the existence of sulfurupon treatment (Table 1, example 2). Also, the molar ratio of N, C, andH in the treated sample is about 1:26:56, which is consistent with thatof didodecyldimethylammonium ion (DDA₊), confirmed the formation ofion-pair ligand (S2⁻-DDA⁺).

TABLE 1 example 2. Elemental analysis results Sample sample weight (mg)N (%) C (%) H (%) S (%) CsPbBr₃ 3.766 0.54 8.16 1.35 0 Treated 5.2620.30 6.26 1.11 0.20 CsPbBr₃

The untreated and treated samples were characterized by X-ray powderdiffraction (XRD). The XRD patterns of both the samples were wellindexed to the standard cubic CsPbBr₃ phase, with no observablesecondary phases (FIG. 31). The instability and degradation of anuntreated QD sample were also substantiated by X-ray fluorescence (XRF)analysis, as indicated by the 1:2:5 atomic ratios of Cs, Pb, and Br,respectively, a considerable deviation from the 1:1:3 ratio expected forperovskites (Table 2, example 2). Moreover, treated samples werenoticeably brighter than the untreated samples under UV light, a clearindicator of the high luminescence obtained after passivation (FIGS.32A-32B). For optical characterizations, the untreated and treatedsamples were spin-coated on a glass substrate to obtain a uniform thinfilm (see Experimental Section). FIG. 23 shows the absorption andphotoluminescence (PL) spectra of the QD films before and after surfacepassivation. It should be noted that the PL signal is quite narrow inboth cases, with an fwhm of 25 nm. Only a very slight change in thebandgap could be observed, as shown in the Tauc plot (FIG. 23, inset),suggesting that the particle size was essentially preserved even 153after the surface treatment. In addition to the characterizationsperformed by electronic spectroscopy, vibrational spectroscopy was usedto probe the specific chemical functionalities on the surface of theQDs. The C═O stretching vibration of OA was used as a specific markermode to probe the ligand exchange process. In this experiment, theabsence of the 1680 cm⁻¹ peak (C═O stretching) in the FTIR spectrum ofthe treated sample (FIG. 33) provided a clear indication of thereplacement of the native oleate functional group by the hybrid ionpair.

TABLE 2 X-ray fluorescence (XRF) results of untreated sample Elem. LineMass[%] 3 sigma Atomic[%] 35 Br K 42.93 0.28 63.06 55 Cs K 14.54 0.4212.84 82 Pb L 42.53 0.3 24.1

To confirm the air and photostability of the passivated CsPbBr₃ QDsfilms (thickness ˜105 nm, FIG. 34A-34B), we tested their propensity forASE through one- and two-photonpumping under ambient conditions (roomtemperature, 60±5% humidity). The results are shown in FIGS. 24A-24B. Ascan be seen, with an increase in the pump fluence, the ASE generated at533 nm shifted to 537 nm, which is consistent with the results of aprevious report.⁴² The onset of stimulated emission in the QD films wasobserved with the immediate increase in the ASE intensity (ASE peak at≈533 nm) and the narrowing of the emission spectrum (fwhm≈4-7 nm, FIG.24A, inset) over the threshold pumping range. A red shift in the ASEpeak (16-21 nm) relative to the PL peak was observed (FIG. 24A). Asshown in FIG. 25A, the ASE threshold fluence for one-photon pumping wasapproximately 192 μJ/cm2, which is lower than the value reported forperovskite thin films⁴³ On the other hand, the threshold obtained forthe QD film in the current study is higher than that recently reportedfor the same QDs, ⁴² probably due to differences in the configurationsof the experimental setups. However, the air and photostabilities of ourtreated QDs are the best reported to date for semi-conductor QDs andeven for perovskite thin films. It is worth noting that the untreatedsample suffered from partial degradation within hours, showing a sharpcontrast in performance, whereas the passivated QD film exhibited thesame threshold and optical characteristics even after 4 months of openair storage and rounds of photostability testing (vide infra and alsosee FIGS. 35A-35B), providing clear evidence of the ultrastability ofperovskite QD films under ambient conditions.

To test the photostability of our QDs films, we measured the variationin ASE intensity as a function of time under continuous femtosecondlaser irradiation in ambient conditions, using a femtosecond lasersystem operated at a repetition rate of 1 kHz. Our results showed thatthere was almost no change in ASE intensity over 1.2×10⁸ laser shots(corresponding to a period of 34 h) for either one- or two-photonpumping (FIGS. 26A-26B). This value substantially exceeds thephotostability of other semiconductor QD systems for which ASE has beenobserved. 44,45

We studied the excited-state dynamics of QDs by the time-correlatedsingle photon counting (TCSPC) technique. At pump fluences below the ASEthreshold (8.5 ρJ/cm², FIG. 36), typical PL lifetime of 11 ns wasobserved. Well above the ASE threshold (220 ρJ/cm²; FIG. 36), an ASElifetime of approximately 2.8 ns was recorded, which further decreasedto 1.9 ns with the increase in pump fluence. Such a fast decay withincreasing pump fluence could be attributed to the dominant contributionfrom the nonradiative decay channel.

We further investigated the pumping of the passivated perovskite QDswith a nonlinear 2PA scheme. Note that 2PA is a third-order nonlinearoptical phenomenon in which a molecule absorbs two photonssimultaneously, resulting in an electronic transition from the groundstate to an excited state via virtual states. 2PA pumping has severaladvantages over 1PA pumping, for instance, minimum risk of photodamageto the sample, longer penetration depth in the absorbing material andthe absence of a phase-matching requirement for the generation andwavelength tuning of coherent light⁴⁵ In addition to these advantages,deleterious effects of the excitation light such as unwanted scatteringand absorption losses are completely suppressed in 2PA.

The ASE-induced in the perovskite QDs during optical pumping wasachieved via 2PA at 800 nm. The following wavelength range highlightsthe distinct advantages offered by 2PA pumping for the perovskite QDsbecause high-powered laser sources are abundant within this range;moreover, the range is also an optical transparency window for water andbiological media. It is worth noting that below and above the thresholdfluence, the fwhm and peak position achieved through 2PA pumping areindistinguishable from those achieved by 1PA excitation; however, adecrease in the intensity of the ASE achieved by 2PA was observed.Femtosecond transient absorption with broadband capabilities shows thatthe dynamics of 2PA-induced ground-state bleaching were similar to thoseof 1PA, and the recovery time fits a single exponential function with acharacteristic time constant of ˜20 ns (FIG. 37)

Although the excited-state dynamics were observed to be identical, 1PAand 2PA ASE showed markedly different thresholds. For two-photonexcitation, the ASE threshold fluence was 12 mJ/cm² (FIG. 25B), which iscomparable to that reported for 2PA ASE in other semiconductor QDs. ⁴⁶Nevertheless, the stability of our passivated perovskite QDs under theapplied operating conditions largely surpasses (photostability unchangedafter 1.2×10⁸ laser shots) all previously reported levels for ASE inQDs, regardless of whether 1PA or 2PA induced that emission. Althoughfew reports on the 2PA pumping of colloidal QDs exist, ^(45,47-49) ASEfrom 2PA pumping in the perovskite family of materials has not beenreported prior to this work.

In summary, a passivation strategy for lead halide perovskite QDs wasdeveloped to alleviate the inherent instability of the material whenoperating under ambient conditions currently is the greatest challengeimpeding the development of perovskite-based devices. Our passivationstrategy is endowed perovskite QDs with unprecedented stability in theair (60±5% humidity) and under high laser fluences, with samples showingno noticeable degradation. An analytical investigation of the passivatedperovskite QDs revealed the formation of a protective layer enrichedwith sulfide (S2⁻-DDA⁺). Because of the ultrahigh stability of theresulting QDs, we were able to induce ultrastable ASE insolution-processed QD films not only by 1PA but also by a 2PA process.ASE in the latter is a phenomenon that has yet to be observed in anyperovskite material. Additionally, our perovskite QD films showedsignificant photostability under continuous pulsed laser excitation inambient conditions for at other colloidal QD systems in which ASE hasbeen observed. The described QD passivation strategy andmultiphoton-induced processes enable the practical implementation ofperovskite QDs and facilitate the exploration of both their linear andnonlinear applications. We believe that this surface passivationmechanism and the validity of the 2PA process will open new avenues ofstudy and hold promise for overcoming the greatest problem precludingthe development of perovskite-based materials for solar cell andnonlinear applications.

EXPERIMENTAL SECTION

Synthesis. 1-Butanol (BuOH, HPLC grade), was purchased from FisherScientific. Oleic acid (OA, technical grade 90%), lead bromide (PbBr2,98%), and octane (98%) were purchased from Alpha Aesar. Sodium sulfidehydrate, cesium carbonate (Cs₂CO₃, 99.995%, metal basis), didodecyldimethylammonium bromide (DDAB, 98%), oleylamine (OLA, technical grade70%), and 1-octadecene (ODE, technical grade 90%) were purchased fromSigma-Aldrich. Toluene (HPLC grade) was purchased from Honeywell Burdick& Jackson. All chemicals were used as procured without furtherpurification. Preparation of Cs-Oleate. Cs₂CO₃ (0.814 g) along with ODE(40 mL) and OA (2.5 mL) were loaded into a 100 mL two-neck flask, driedfor 1 h at 120° C., and then heated under N₂ at 150° C. until all Cs₂CO₃had reacted with OA. The solution was maintained at 150° C. beforeinjection to prevent the solidification of Cs-oleate.

Preparation of S Precursor (S²⁻-DDA⁺). Similarly to Jiang's method, 41.3mL of toluene containing 0.15 m mol of DDAB was mixed with 3 mL of 50 mMaqueous Na₂S solution. The S2⁻ anions were then transferred from theaqueous phase to the toluene phase. The toluene phase was separated andused as a sulfur precursor (DDA-S2⁻) in subsequent experiments.

Synthesis of CsPbBr₃ QDs. In contrast to the methods reportedelsewhere^(27,32) in the current study, the OLA and OA were notpredried, and not all solvents used were anhydrous. ODE (125 mL) andPbBr₂ (1.725 g) were loaded into a 500 mL two-neck round-bottom flaskand dried under vacuum for 1 h at 120° C. OLA (12.5 mL) and OA (12.5 mL)were then injected at 120° C. under N₂. After the PbBr₂ salt hadcompletely dissolved, the temperature was raised to 180° C., andCs-oleate solution (10 mL, prepared as described above) was quicklyinjected; an ice-water bath was then used to cool the reaction mixtureafter 5 s.

Selected Purification of CsPbBr₃ QDs. The crude solution was cooled in awater bath and directly transferred to centrifuge tubes. Aftercentrifuging the tubes at 7000 rpm for 15 min, the supernatant andprecipitate were collected separately. The supernatant was mixed withBuOH for centrifugation, and the bottom sample was then collected andredissolved in toluene (sample 1). The precipitate was dispersed byadding toluene to collect the new supernatant (sample 2) aftercentrifugation and redisperse the new precipitate in toluene (sample 3).

Treatment of CsPbBr₃ QDs and Preparation of CsPbBr₃ QDs Films. To 1 mLof the different CsPbBr₃ QDs (15 mg/mL), 50 μL of OA was added understirring. A certain amount of sulfur precursor was then addedsequentially. The sample was precipitated with twice the amount of BuOHand redissolved in 200 μL of octane. Thin films of CsPbBr₃ QDs wereobtained by spin-coating the treated CsPbBr₃ QD solution under ambientconditions onto glass substrates. Moreover, the untreated sample waswashed with BuOH alone and redissolved in octane to fabricate the thinfilms. The glass substrates were cleaned by standard procedure withdetergent, deionized water, acetone, and isopropanol. The cleanedsubstrates were treated with plasma for 5 min before depositing the QDfilms. Highly smooth, densely packed and pinhole-free thin films wereobtained by spin-coating at 500 rpm (10 s) and then at 1500 rpm (40 s).

Characterization. UV-vis absorption spectra were obtained using anabsorption spectrophotometer from Ocean Optics. Carbon, hydrogen,oxygen, and sulfur analysis was performed using a Flash 2000 elementalanalyzer (Thermo Fischer Scientific). Photoluminescence was tested usingan FLS920 dedicated fluorescence spectrometer from EdinburghInstruments. Quantum yield was measured using an Edinburgh Instrumentsintegrating sphere with an FLS920-s fluorescence spectrometer. FTIR wasperformed using a Nicolet 6700 FT-IR spectrometer. Powder XRD patternswere recorded using Siemens diffractometer with Cu Kα radiation(λ=1.54178 Å). TEM analysis was carried out with a Titan TEM (FEICompany) operating at a beam energy of 300 keV and equipped with aTridiem postcolumn energy filter (Gatan, IQD). The samples were imagedin EFTEM mode with a 20 eV energy slit inserted around thezero-energy-loss electrons to acquire high-resolution TEM (HRTEM)micrographs. The spatial distribution of the elements Pb and S wasdetermined and acquired using the EFTEM technique by selecting thePbO-edge (86 eV) and S L-edge (165 eV) in the three-window mappingmethod. Morphological investigations and cross-sectional imaging of theQD films were carried out on a Karl Zeiss FESEM.

Experimental Details of Optical Pumping, Single Photon Counting, andTransient Absorption. All ASE pumping experiments were conducted at roomtemperature. The 1PA pumping experiments were performed using afemtosecond laser system operated at a wavelength 800 nm with 35 fspulses and a repetition rate of 1 kHz. UV pump pulses at 400 nm wereobtained in a straightforward manner by the second harmonic (frequencydoubled) of the fundamental beam, where 100 μJ of the laser output wasfocused in a 100 μm BBO nonlinear crystal. The 2PA pumping experimentswere carried out by directly using the fundamental beam at 800 nm.

TCSPC for lifetime measurements was performed using a Halcyone MCmultichannel fluorescence up-conversion spectrometer (Ultrafast Systems)at an excitation wavelength of 400 nm. Nanosecond transient absorptionexperiments were conducted using an EOS setup (Ultrafast Systems).Detailed information regarding the experimental setup has been publishedelsewhere. ^(50,51)

Supporting Information

EF-TEM characterization was conducted on sulfur-treated NCs. Elementalmapping was carried out for lead and sulfur. Results indicate a highcorrelation between lead and sulfur (FIG. 30B, 30C), suggesting thatsulfur was indeed present on the NCs as a capping layer rather than as aPbS shell.

Both the treated and untreated samples were washed with 1-butanol,centrifuged and placed under vacuum. The color varied and theluminescence was improved with the applied treatment. X-ray fluorescence(XRF) was used to analyze the major elements of the sample. The Cs:Pb:Brratio was approximately 1:2:5 for the untreated sample, confirming theinstability of the inorganic perovskite, which is consistent with theXRD pattern (FIG. 31). The elemental ratio of the treated sampleobtained by XRF analysis is not reasonable, perhaps because the sulfur Kline (2.307 keV) is very close to the lead M line (2.42 keV) and becausehybrid ion pair ligand passivation was performed. However, the XRDpattern (FIG. 31) of the treated sample corresponds to cubic phaseCsPbBr₃, indicating high stability in air (60% humidity in the lab,KAUST, Saudi Arabia).

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A 2014, 118, 3090-3099.

Example 3

Lead halide perovskites have recently emerged as promising candidatematerials for optoelectronic applications such as photovoltaics,^([1-3])lasing,^([4-6)] and photodetectors,^([7-9]) due to their size-tunableoptical bandgaps, attractive absorption, narrow emission, andextraordinary charge transport properties. These impressivecharacteristics have also triggered intense interest in applyingperovskites to the field of light-emitting diodes (LEDs).^([10])However, perovskite LEDs (PeLEDs) still exhibit overall low performancein comparison to other materials technologies, such as Cd-based quantumdots (QDs).^([11-13]) Moreover, recent advances in integrating leadhalide perovskites in PeLEDs have been mainly limited to hybridorganic-inorganic perovskites, such as CH₃NH₃PbBr₃.^([14,15]) Thehighest performance so far achieved was obtained for a green PeLED withCH₃NH₃PbBr₃ using a self-organized conducting polymer anode exhibiting acurrent efficiency of 42.9 cd A⁻¹ and an external quantum efficiency(EQE) of up to 8.53%.^([16]) Unfortunately, such hybridorganic-inorganic perovskite materials and their resultant devices arehampered by their limited stability.^([17-19])

All-inorganic perovskite QDs (APQDs), such as CsPbX₃ (X=Cl, Br, and I),exhibit superior thermal stability compared to their hybrid analogues.They have the potential to be integrated into various optoelectronicdevices that can exploit their quantum confinement effects. Kovalenkoand co-workers fabricated CsPbX₃ QDs with exceptionally tunable opticalproperties and high photoluminescence (PL) quantum yield, suggesting amajor opportunity to employ this family of materials for LEDs.^([20])Unfortunately, the highest EQE reported so far is 0.19%,^([21]) which ispartly due to the QDs being capped with relatively insulating longligands that are required for the processing and stability of theQDs.^([22,23]) Replacing these long ligands (usually oleylamine (OAm)and oleic acid (OA)) with shorter ligands without degrading ordestabilizing the APQD films remains the key challenge preventing thefabrication of efficient LEDs from APQDs.

Here, we realize highly stable films of CsPbX₃ QDs capped with a halideion pair (e.g., di-dodecyl dimethyl ammonium bromide, DDAB), arelatively short ligand that facilitates carrier transport in the QDfilm and ultimately enables us to fabricate efficient PeLEDs. Thesynthesis of these films was only possible through the design of aligand-exchange strategy that includes an intermediate step to desorbprotonated OAm, which otherwise would result in the degradation of APQDsthrough a direct conventional ligand-exchange route. As a result of ournovel ligand exchange strategy, we were able to utilize halide ionpair-capped CsPbBr₃ QDs in green PeLED with a device structure ofITO/PEDOT: PSS/PVK/QDs/TPBi/LiF/AI, achieving an EQE of 0.65% with aluminance of 165 cd m⁻² at a voltage of 7.5V, which is much highercompared to the APQD LEDs without ligand exchange. Furthermore, wedemonstrated the flexibility and generality of our ligand exchangestrategy by exploiting mixed halide ion pairs to tune the emission ofthe QDs and further to fabricate blue PeLEDs possessing an EQE of 0.18%with a maximum luminance of 35 cd m⁻² at a voltage 7.5 V. The reportedefficiencies for both green and blue PeLEDs in this work represent amajor leap for the family of APQD materials and pave the way to furthertheir exploitation in optoelectronics through judicious surfaceengineering.

The APQDs were synthesized via a modified hot-injection synthesisstrategy by injecting cesium oleate into a PbBr₂ solution at 180° C. andstirring for 5 sec.^([24]) The as-obtained reaction mixture containingQDs was quenched in an ice bath and purified for further treatment (seeExperimental Section for details). These purified QDs (herein referredas P-QDs) are soluble in nonpolar solvents such as toluene due to thepresence of organic ligands (i.e., OA and OAm) on the QD surface. Thesolution of P-QDs/toluene exhibits a bright green color (FIG. 43), whichwill turn into light brown immediately with the addition of OA,suggesting instability problems with the presence of excess OA andpossible formation of large aggregates (the products were referred asOA-QDs). However, upon the introduction of DDAB in the final treatmentstep, a luminous bright green color reappeared (the products werereferred to as DDAB-OA-QDs) (FIG. 43).

High-resolution transmission electron microscopy (HRTEM) was used totrack the morphological changes during the treatment procedures. Asshown in FIG. 38A, the P-QDs are cubic shaped and monodisperse, with anaverage size of 10 nm (FIG. 38A). The OA-QDs were washed with butanoland re-dispersed in toluene after removal of precipitates viacentrifugation for TEM characterization. For the OA-QDs that werecleaned immediately after the addition of OA, TEM shows an increasedaverage particle size (FIG. 38B), while more obvious size increment canbe observed if the QDs were kept with OA for 30 min before furthercleaning (FIG. 38C). However, the particle sizes and shapes can bepreserved with immediate DDAB treatment after the addition of OA (FIG.38D). X-ray diffraction (XRD) confirmed the cubic crystal phase of allthe samples, in accordance with previous reports (FIG. 44).^([25,26])

FIG. 38E shows the UV-Visible absorption spectra of the QDs before(P-QDs) and after (DDAB-OA-QDs) ligand exchange. The close match of thetwo spectra implies that the size of QDs was preserved during ligandexchange. The enhanced PL intensity at 513 nm, along with the quantumyield increasing from 49% to 71% for DDAB-OA-QDs, indicate a betterpassivation of the surface trap states.^([27]) Notably, a 4 nm shift inPL spectra was noticed after OA treatment, possibly due to theadsorption of OA on the QD surface or the formation of slightly largerparticles.

To elucidate the ligand exchange process, Fourier transform infraredspectroscopy (FTIR) was used to examine the presence of ligand types inthe as-synthesized and treated QD samples. As displayed in FIG. 38F, allthe samples show CH₂ and CH₃ symmetric and asymmetric stretchingvibrations in the range of 2840-2950 cm⁻¹, and CH₂ bending vibration at1466 cm⁻¹, which are the typical absorption bands for species withhydrocarbon groups. ^([28]) The FTIR spectrum for the P-QDs sample showsan N—H stretching mode at 3300 cm⁻¹, indicating the presence of OAm onthe P-QD surface.^([29]) The strong signal at 1635 cm⁻¹ can be assignedto the asymmetric NH₃+ deformation,^([30]) consistent with theappearance of N 1S at 401.8 eV in the XPS analysis (discussed below).The absorptions at 1605, 1535 and 1406 cm⁻¹ are ascribed to twoasymmetric vibrations and one symmetric stretching vibration of thecarboxylate group, indicating that oleate anions are complexed on the QDsurface.^([31,32]) With the addition of OA, OAm gets protonated byexcess protons, and further reacts with deprotonated OA to form anacid-base complex, resulting in their desorption from the QDsurface.^(33,34) Subsequently, the excess OA adsorbs on the QD surfaces,as can be inferred from the emergence of the characteristic C═Ostretching vibration band at 1710 cm⁻¹ in the IR spectrum of the OA-QDsample.^([35]) To shed more light on the ligand exchange process, wequantified the zeta potentials of various QD samples. In contrast with anegative value of −16 mV for the P-QDs, the OA-QDs shows a positivevalue of 10 mV, confirming the adsorption of OA on the QD surface andresulting in a polarity change. Such a scenario of protonation anddesorption of OAm through the addition of OA has also been reportedpreviously. ^([36,37])

However, both characteristic bands of N—H in OAm and C═O in OA disappearin the IR spectrum of the DDAB-OA-QDs, indicating a complete exchange ofOA by DDAB on the QD surface. Such an exchange process may arise fromthe stronger affinity of Br-ions to the positive sites (Pb²⁺ or Cs⁺) onthe QD surface compare to that of oleate group,^([22,38]) as well as thestronger affinity of DDA⁺ with the negative sites (Br) or adsorbed Br onthe QD surface In addition, due to the larger steric hindrance of thebranched structure of DDA⁺, fewer DDA⁺ ions may be adsorbed onto the QDsurface, inducing a more negatively polarized QD due to the Br-richsurface,³⁹ which is confirmed by a zeta potential value of −60 mV. Sucha large zeta potential along with the large steric hindrance of DDA⁺ensure a higher stability of the DDAB-OA-QD solution (FIGS. 34A-34B).Such a passivation strategy, therefore, offers a solution to theinstability problems caused by the highly dynamic binding of OAm and OAwith the QDs,^([34]) so as to achieve a more stable ligand capping ofthe QDs through an X-type^([22,23]) binding using the ion pair ligand.

We also probed into the surface composition of all of the QD samplesusing XPS. The representative survey spectrum of P-QDs confirms thepresence of Cs, Pb, Br, C, N, and O elements (FIG. 39A).³⁹ Nosignificant changes were observed for the Cs 3d, Pb 4f, Br 3d corelevels spectra for the three samples (FIG. 46), yet the high-resolutionspectra of the N 1s core level obtained from the three samples shownoticeable changes (FIGS. 39B-39D). The N 1s core level for P-QDs wasfitted with two components at 399.9 eV and 401.8 eV. The dominant peakat 401.8 eV corresponds to protonated amine groups (—NH₃+) while thepeak at 399.9 eV is attributed to amine groups.^(40,41) The N 1s corelevel for OA-QDs was fitted with similar components with reducedintensity. The existence of two N 1s core level suggested severalequilibria from ammonium to amine that could exist.³⁴ However, the N 1score level for DDAB-OA-QDs was fitted with a single peak at 402.2 eVcorresponding to tert-ammonium cations from di-dodecyl dimethylammoniumbromide.^(42,43)

Combining the above analysis, we hypothesize that the acidic protonsfacilitate the removal of the OAm ligand by protonation. The protonatedOAm ligands subsequently form acid-base complex with deprotonated OAgroups,^([36,37]) and promote the coordination of the Br anions with thepositively charged surface metal centers (Cs⁺ or Pb²⁺), as schematicallyillustrated in FIG. 40, while the existence of DDA⁺ on the QD surfacehelps to maintain their solubility in toluene. It is important to notethat the intermediate process of adding excess amount of OA ligands inour two-step ligand exchange procedure is vital to avoid the formationof 2D quantum wells of (OAm)PbBr₄ (FIG. 47),^([44]) likely through aprotonation salt formation process. This is further confirmed by thetime-dependent decrement in PL intensity of the as-synthesized P-QDs(FIG. 48) and clear morphological change from well-defined cubic shapesto larger plate-like structures (FIGS. 49A-49D).

The surface morphology of the spin-coated thin films of P-QDs andDDAB-OA-QDs exhibit the densely packed surfaces over larger areas (FIGS.50A-50B). The P-QDs film showed larger grains compared to theDDAB-OA-QDs films further confirming the effect of surface-treatment instabilizing the QDs. More importantly, the smoother films obtained inthe case of DDAB-OA-QDs samples can correspond to the passivation ofsurface trap states as observed from the UV-Vis absorption spectra. Theabsorbance measurements carried out on QDs used for device fabricationare shown in FIGS. 41A-41D. Band gaps estimated from the Tauc plots(FIG. 41A inset) showed a slight change of 0.04 eV upon treatment withOA and DDAB. The energy levels (conduction/valence band) estimated fromthe PESA measurement and band gaps respectively shown in FIG. 41B are ofinterest for optoelectronic engineering of PeLEDS. These all-inorganicPeLED devices were characterized using the cross-section TEM (FIG. 41C)where the multiple layers are arranged in the sequential order: indiumtin oxide (ITO), poly(ethylene dioxythiophene):polystyrene sulfonate(PEDOT:PSS, 40 nm), poly(9-vinlycarbazole) (PVK, 20 nm), perovskite QDs(8 nm), 2,2′,2″-(1, 3, 5-benzenetriyl) tris-[1-phenyl-1H-benzimidazole](TPBi, 42 nm), and LiF/AI (10/100 nm). PVK is used as ahole-transporting and electron-blocking layer while TPBi is employed asan electron-transporting layer. The PVK layer reduces the hole-injectionbarrier, blocks the electrons in the active layer, and hence allow theholes and electrons to recombine effectively in the QD emittinglayer.^([21]) The complete device architecture used for the currentstudy of is shown schematically in FIG. 41D.

The current density-luminance-voltage (J-L-V) characteristic of thePeLEDs based on the typical green CsPbBr₃ QDs (DDAB-OA-QDs) is presentedin FIG. 42A. The turn-on voltage for the PeLED device is 3.0 V,significantly lower than that of the PeLED based on untreated QDs(P-QDs) as emitting layer (FIG. 51A-51B), indicating that an efficient,barrier-free charge injection into the QD emitters wasachieved.^([45,46]) The luminance intensifies as the voltage increase,achieving the maximum value of 330 cd m⁻² under a forward bias of 9 V.The current efficiency (CE) and EQE as a function of voltage for atypical green PeLEDs are shown in FIG. 42B. At the voltage of 7.5 V, apeak EQE value of 0.65% was reached with a luminance of 165 cd m⁻²,while an EQE of 0.001% with a luminance of 0.38 cd m⁻² was achieved forthe control device using P-QDs as emitting layer (FIGS. 51A-51B). Thelarge enhancement in EQE indicates that the charge carrier balance inthe QDs layer was significantly improved due to the use of halide ionpair ligands for passivation. The normalized electroluminescencespectrum (EL) of the QLEDs is shown in FIG. 42C. The device gives anemission peak at 515 nm with a very narrow emission peak with full-widthwavelength at half maximum (FWHM) of 19 nm, which is attributed to thenarrow band-edge emission of QDs with a slightly red-shifted emissioncompared with the PL spectrum acquired from the QD solution. Strikingly,the parasitic emission originated from the charge transport layers(i.e., TPBi or PVK) was undetectable in the entire EL spectrum undervarious voltages, indicating good electron and hole blocking functionsof both PVK and TPBi layers. The devices emit bright and uniform greenlight from the whole pixel under a bias of 5 V as shown in the inset inFIG. 42C.

The blue PeLED devices were fabricated by using the same architecture ofITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/AI, wherein the blue QDs were fabricatedvia an anion exchange strategy^([47,48]) by treating the P-QDs with amixed halide ion pair ligand (di-dodecyl dimethyl ammonium bromidechloride, DDABC, see experimental section). The J-L-V characteristic ofthe blue PeLEDs (CsPbBr_(x)Cl_(3-x)) is presented in FIG. 42D. Theturn-on voltage (calculated at a luminance of 1 cd m⁻²) required for theblue PeLED device is 3.0 V, lower than the previous reported PeLEDvalue,^([21]) also employing an efficient and barrier-free chargeinjection into the QD emitters.^([45,46]) The electroluminescenceintensifies as the voltage is increased, which leads to the maximumvalue of 35 cd m⁻² under the applied voltage of 7.5 V (FIG. 42E). Theperformance of these APQD LEDs can be further optimized in the future bycontrolling the crystal phase, balancing charge transport, improving PLefficiency, as well as by other short ligand replacement. The normalizedEL spectra of blue PeLEDs with emission wavelength peaks at 490 nm(FWHM=19 nm) is shown in FIG. 42F. The devices exhibited a saturated andpure color as shown in the insert image of FIG. 42F.

In summary, we demonstrated a two-step ligand exchange process forpassivating CsPbX₃ (X=Br, Cl) QDs with halide and mixed halide ionpairs. Blue and green PeLEDs based on the passivated CsPbX₃ QDsdisplayed a sharp EL peak (FWHM=19 nm) with a maximum luminance of 35 cdm⁻² for the blue and 330 cd m⁻² for the green. The resultant PeLEDs'performance demonstrates that complete ligand exchange by desorption ofprotonated OAm and subsequent treatment with a halide ion pair ligandimproves charge carrier balance and device EQE. Our findings pave theway for the development of PeLEDs of high EQE and high luminescencebased on stable inorganic perovskite QDs.

EXPERIMENTAL

Materials

1-butanol (BuOH, HPLC grade), was purchased from Fisher Scientific.Oleic acid (OA, technical grade 90%), lead bromide (PbBr₂, 98%) andoctane (98%) were purchased from Alpha Aesar. Cesium carbonate (Cs₂CO₃,99.995%, metal basis), didodecyldimethylammonium chloride (DDAC, 98%),didodecyldimethylammonium bromide (DDAB, 98%), oleylamine (OAm,technical grade 70%), and 1-octadecene (ODE, technical grade 90%) werepurchased from Sigma-Aldrich. Toluene (HPLC grade) was purchased fromHoneywell Burdick & Jackson. All chemicals were used as procured withoutfurther purification.

Preparation of Cesium Oleate Solution

Cs₂CO₃ (0.814 g) was loaded into 100 mL 2-neck flask along with ODE (30mL) and oleic acid (2.5 mL), dried for 1 h at 120° C., and then heatedunder N₂ to 160° C. until all Cs₂CO₃ reacted with OA. The solution waskept at 160° C. to avoid solidification before injection.

Synthesis and Purification of CsPbBr₃ QDs^([24])

100 mL of octadecene (ODE), 10 mL of OAm, 10 mL of OA, and PbBr₂ (1.38g) were loaded into a 250 mL flask, degassed at 120° C. for 30 min andheated to 180° C. under nitrogen flow. 8 mL of cesium oleate solution(0.08 M in ODE) was quickly injected. After 5 s, the reaction mixturewas cooled using an ice-water bath. The crude solution was directlycentrifuged at 8000 rpm for 10 min, the precipitate was collected anddispersed in toluene. One more centrifugation was required for purifyingthe final QDs.

Treatment of CsPbBr₃ QDs

1 mL of the purified CsPbBr₃ QDs (15 mg/mL), 50 μL of OA was added understirring, then added 100 μL DDAB toluene solution (0.05 M). The mixturesolution was precipitated with BuOH after centrifugation and redissolvedin 2 ml of octane. For the blue QDs (CsPbBr_(x)Cl_(3-x)), a similartreatment procedure was applied except a mixed halide ion pair ligand (3ml 0.005M KBr aqueous solution mixed with 3 ml 0.05 M DDAC toluenesolution, top layer solution was collected after centrifugation).

Device Fabrication

PEDOT:PSS solutions (Clevios™ PVP Al4083, filtered through a 0.45 μmfilter) were spin-coated onto the ITO-coated glass substrates at 4000rpm for 60 s and baked at 140° C. for 15 min. The hole transporting andelectron blocking layer were prepared by spin-coating PVK chlorobenzenesolution (concentration: 6 mg mL⁻¹) at 4000 rpm for 60 s. Perovskite QDswere deposited by spin-coating at 2000 rpm for 60 s in air. TPBi (40 nm)and LiF/AI electrodes (1 nm/100 nm) were deposited using a thermalevaporation system through a shadow mask under a high vacuum of 2*10⁴Pa. The device active area was 6.14 mm² as defined by the overlappingarea of the ITO and Al electrodes. All the device tests were done underambient condition.

Characterization

UV-Vis absorption spectra were obtained using an absorptionspectrophotometer from Ocean Optics. Carbon, hydrogen, oxygen, andsulfur analysis was performed using a Flash 2000 elemental analyzer(Thermo Fischer Scientific). Photoluminescence was tested using anFLS920 dedicated fluorescence spectrometer from Edinburgh Instruments.Quantum yield was measured using an Edinburgh Instruments integratingsphere with an FLS920-s fluorescence spectrometer. FTIR was performedusing a Nicolet 6700 FT-IR spectrometer. Powder X-ray diffraction (XRD)patterns were recorded using Siemens diffractometer with Cu Kα radiation(λ=1.54178 Å). TEM analysis was carried out with a Titan™ TEM (FEICompany) operating at a beam energy of 300 keV and equipped with aTridiem™ post-column energy filter (Gatan, IQD.). The SEM investigationswere carried out on the Carl Zeiss, Gemini column FESEM. Photoelectronspectroscopy in air (PESA) measurements were carried out on the thinfilm samples, using a Riken Photoelectron Spectrometer (Model AC-2). Thepower number was set at 0.3. XPS studies were carried out in a KratosAxis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-raysource (hv=1486.6 eV) operating at 150 W, a multi-channel plate anddelay line detector under 1.0×10⁻⁹ Torr vacuum. Measurements wereperformed in hybrid mode using electrostatic and magnetic lenses, andthe take-off angle (angle between the sample surface normal and theelectron optical axis of the spectrometer) was 0°. All spectra wererecorded using an aperture slot of 300 μm×700 μm. The survey andhigh-resolution spectra were collected at fixed analyzer pass energiesof 160 and 20 eV, respectively. Samples were mounted in floating mode inorder to avoid differential charging^([49]). Charge neutralization wasrequired for all samples. Binding energies were referenced to the C 1speak (set at 284.8 eV) of the sp3 hybridized (C—C) carbon fromoleyalmine and oleic acid. The data were analyzed with commerciallyavailable software, CasaXPS. The individual peaks were fitted by aGaussian (70%)-Lorentzian (30%) (GL30) function after linear orShirley-type background subtraction. The EL spectra and luminance(L)—current density (J)—voltages (V) characteristics were collected byusing a Keithley 2400 source, a calibrated luminance meter (KonicaMinolta LS-110), and a PR-705 SpectraScan spectrophotometer (PhotoResearch) in the air and at room temperature. Zeta potentialmeasurements were performed using a Zetasizer Nano-ZS (MalvernInstruments). Each sample was measured 5 times and the average data waspresented.

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When the P-QDs were mixed with DDAB directly, XRD spectrum reveals thedefinite formation of a single lamellar compound (shown in FIG. 47),which is consistent with the previous report. The QDs PL intensitydecreased continuously together with a blueshift of PL peak positionwith time until it reached 506 nm, and the emission color disappearedafter 2.5 h. (FIG. 48) Possibly due to the free oleyamine (OAm) existedinside the solution after DDAB treatment, (OAm)₂PbBr₄ could be formedgradually causing the original CsPbBr₃ emission decreased timely, whichwas supported by the TEM images (no original cubic shape was reserved)(FIGS. 49A-49D).

It should be noted that ratios, concentrations, amounts, and othernumerical data may be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a concentration range of “about0.1% to about 5%” should be interpreted to include not only theexplicitly recited concentration of about 0.1 wt % to about 5 wt %, butalso include individual concentrations (e.g., 1%, 2%, 3%, and 4%) andthe sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within theindicated range. In an embodiment, “about 0” can refer to 0, 0.001,0.01, or 0.1. In an embodiment, the term “about” can include traditionalrounding according to significant figures of the numerical value. Inaddition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about‘y’”.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations, andare set forth only for a clear understanding of the principles of thedisclosure. Many variations and modifications may be made to theabove-described embodiments of the disclosure without departingsubstantially from the spirit and principles of the disclosure. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure.

What is claimed:
 1. A film comprising: a passivated perovskite quantumdot including: a core of the form APbX₃, where A is Cs⁺, Rb⁺, CH³NH₃ ⁺,or HC(NH₂)₂ ⁺, and X is a halogen; and a capping ligand including aninorganic-organic hybrid ion pair including di-dodecyl dimethylammoniumand at least one anion selected from the group of Cl⁻, Br⁻, I⁻, SH⁻,sulfide (S²⁻), Se²⁻, HSe⁻, Te²⁻, HTe⁻, TeS₃ ²⁻ or AsS₃ ²⁻.
 2. The filmof claim 1, wherein the inorganic-organic hybrid ion pair includesdi-dodecyl dimethylammonium sulfide (S²⁻-DDA⁺).
 3. The film of claim 1,wherein the core of the passivated perovskite quantum dot isCsPbCl_(3-x)Br_(x), where x is 0 to
 3. 4. The film of claim 1, whereinthe core of the passivated perovskite quantum dot is CsPbCl₃.
 5. Thefilm of claim 1, wherein the core of the passivated perovskite quantumdot is CsPbBr₃.
 6. The film of claim 1, wherein the at least one anionis Cl⁻, Br⁻, or I⁻.
 7. The film of claim 1, wherein theinorganic-organic hybrid ion pair includes di-dodecyl dimethyl ammoniumbromide (Br⁻-DDA⁺).
 8. The film of claim 1, wherein theinorganic-organic hybrid ion pair includes di-dodecyl dimethyl ammoniumchloride (Cl⁻-DDA⁺).
 9. The film of claim 1, wherein theinorganic-organic hybrid ion pair includes di-dodecyl dimethyl ammoniumiodide (I⁻-DDA⁺).
 10. The film of claim 1, wherein a diameter of thepassivated perovskite quantum dot ranges from about 75 nanometer (nm) to160 nm.
 11. The film of claim 1, wherein a diameter of the core of thepassivated perovskite quantum dot ranges from about 5 nm to 20 nm. 12.The film of claim 1, wherein a diameter of the core of the passivatedperovskite quantum dot ranges from about 6 nm to 16 nm.
 13. The film ofclaim 1, wherein a thickness of the capping ligand ranges from about 70nm to 140 nm.
 14. The film of claim 1, wherein a thickness of thecapping ligand ranges from about 90 nm to 120 nm.
 15. The film of claim1, wherein the passivated perovskite quantum dot has a photoluminescencequantum yield (PLQY) of about 70% or more.
 16. An optoelectronic devicecomprising the film according to claim
 1. 17. The optoelectronic deviceof claim 16, wherein the optoelectronic device is a lasing device. 18.The optoelectronic device of claim 16, wherein the optoelectronic deviceis a quantum dots light emitting device (QLED).
 19. The optoelectronicdevice of claim 18, further comprising an electron transport layer and ahole transport layer.
 20. The optoelectronic device of claim 16, whereinthe optoelectronic device is a photovoltaic device.